The present invention is related to a separator and a method of separation. The separator and method find particular use in the separation of multi-phase mixtures, especially those containing gases, liquids and solids. The present invention is particularly suitable for, but not limited to, application in the separation of the products of oil and gas wells on land, on a platform, or especially in subsea locations.
The conventional approach to for the development of subsea hydrocarbon producing fields is to establish a plurality of subsea wells connected through a subsea infrastructure, pipelines and risers to a surface processing facility. As a result, solids in the well flow are currently carried in the fluid to the surface processing facility. The surface processing facility may be a floating production vessel or a platform. The surface processing facility has typically included separation equipment, to separate sand, gas and water from the oil produced from the wells. Gas and water recovered in this way are frequently reinjected at the seabed into the wells. This necessarily entails pumping the gas and water back down a series of pipelines and risers to the seabed.
A new approach to this problem has been to dispense with much of the processing facilities at the surface and to locate the relevant installations on the seabed at a location adjacent to or convenient to the production wells. The object is to remove the extensive infrastructure needed to bring all the produced streams to the surface and to return significant portions to the seabed for reinjection. While such an arrangement indeed reduces the capital expenditure and operating costs of the installations, it is accompanied by significant problems.
With processing equipment, such as water, gas and solid separators, situated on surface vessels or platforms, the servicing and maintenance of the components is relatively straightforward, with access to the equipment being readily available. However, this is not the situation with subsea installations, in particular those beyond safe diver depth. Rather, access to the equipment located on the seabed can be by either robotic and remotely controlled equipment or by retrieving equipment to the surface for repair and/or servicing. Therefore, intervention is very limited, with the ability to access the equipment decreasing as the depth of the installation below the surface increases. It will thus be appreciated that the failure of equipment located on the seabed represents a major operational problem and a high intervention expenditure. Consequently, the frequency of component failure must be minimised, in particular if the option of a subsea installation is to be economically viable.
A particular problem arises as a result of solids produced from subsea wells. It is frequently the case that subsea wells produce a significant quantity of solid formation material, together with the various fluid phases. The solid formation materials include coarse, medium and fine solid particles, such as grit and sand, together with very fine particles, such as scale or silt. In some cases, the solids produced may include stones, gravel and small rocks, depending upon the operation being performed downhole and the nature of the subterranean formation. Entrained solids leaving the wellhead are responsible for a high rate of equipment erosion and destruction. It is known to provide surface installations with equipment for removing produced solids. Typically, the equipment employed is one or more settling vessels. The nature of solids settling necessarily requires that such vessels are very large in size. It would be advantageous if an effective means of separating produced solids from the fluid phase could be found that did not rely on such large equipment.
Separation installations are installed downstream of the wellhead and operate at pressures significantly below the pressure of the fluid produced at the wellhead. This necessitates employing one or more chokes immediately downstream of the wellhead, in order to reduce the fluid pressure to the operating pressure of the settling equipment. However, choke components are particularly prone to damage from entrained solids. Further, the failure of the wellhead choke represents a major operational problem, in particular if the choke must be replaced, an operation that requires the separation system to be shut down until the repair or replacement can be carried out, resulting in lost production.
Accordingly, there is a need for an effective system and method for separating entrained solids from fluids produced from the well upstream of the wellhead choke, that is at wellhead pressures.
It is known to provide surface installations with desanders. The known desanders comprise a single cyclone insert housed inside a vessel. The desanders of this kind may be employed either upstream or downstream of the wellhead choke and can thus operate at wellhead pressures. While this arrangement represents an improvement over the arrangements discussed above, a number of significant problems remain. First, the known desanders rely upon conventional cyclone technology. The separation efficiency of conventional cyclone separators is at best only moderate, as discussed in more detail below. In particular, conventional cyclone separators generally have a narrow range of operating fluid flowrates. While acceptable in many applications, this represents a major restriction in their suitability for use in subsea installations. It is desirable for a subsea installation to have a wide operational range to handle the daily changes in operating conditions, and also be capable of meeting the well's production profile over an operating lifetime of from 5 to 10 years. During this time, the composition and flowrate of the fluid produced from a given subsea well will vary, sometimes greatly. In is unacceptable to install in such a situation a system that has a narrow window of operation. Accordingly, there is a need for a system that can operate for an extended period of time over a wide range of fluid flowrates and compositions.
Second, the known wellhead separators are designed for the separation of sand, as suggested by their name. However, it is frequently the case that a well will produce a wide range of solid material, much of which is significantly larger in size than sand. A desander, by design, is optimised to separate sand from the produced fluids. However, due to the limitations of conventional cyclone separation techniques, this system will be inefficient in the separation of larger sized or coarser solids, allowing the larger solid particles to pass the separator and enter the downstream equipment, with the results discussed above. Accordingly, there is a need for a separation system that can handle a wide range of solid particle sizes and maintain a high efficiency of separation.
Finally, as soon as fluid is produced from a well, the various phases, including liquid phases and gas, begin to separate. This natural separation of the various phases in the produced fluid stream is advantageous and assists the downstream separation operations. It would be most desirable if a system could be provided that efficiently separates a wide range of solid particles over a wide range of operating pressures and flowrates, but without subjecting the fluid to a high rate of shear causing the already separated phases to mix and emulsify.
US 2004/0217050 discloses a solids separation system for well flowback for use in a subsea wellhead installation. The system uses a hydrocyclone to intermittently separate the heavy phase containing solids from production fluid. The lighter phases are transported to the surface for separation using conventional techniques. The heavy phase collects in the bottom of the hydrocyclone, from where it is periodically removed through a standard choke assembly, to reduce its pressure. The system of US 2004/0217050 does not fully address and resolve the aforementioned issues. Accordingly, there is a need for an improved separation system, suitable for use in a subsea environment, that addresses the aforementioned problems.